Compound A (11-oxo-12-alkoxy-8,9-anhydroerythromycin A 6,9-hemiketal) which is an erythromycin compound represented by the formula: wherein R′ represents a lower alkyl group, is a compound useful as a synthetic raw material of a gastro-prokinetic agent (for example, see de(N-methyl)-11-deoxy-N-isopropyl-12-O-methyl-11-oxo-8,9-anhydroxyerythromycin A 6,9-hemiacetal (GM-611), New Current 7(13), pp. 19-21, issued on Jun. 10, 1996). A method for producing the erythromycin compound has been already known. Of these, from erythromycin A represented by the formula: 2′-O-acetylerythromycin A represented by the formula: is obtained, and then, via 2′-O-acetyl-4″-O-formylerythromycin A represented by the formula: to produce 2′-O-acetyl-4″-O-formyl-8,9-anhydroerythromycin A 6,9-hemiketal represented by the formula: and finally the above-mentioned Compound A is obtaind as disclosed in Japanese Provisional Patent Publication No. 100291/1997 (which corresponds to U.S. Pat. No. 5,959,088). In this method, however, 11-O-formyl-2′-O-acetyl-4″-O-formylerythromycin A is by-produced in the formylation step, and the by-product is then transformed to a hemiketal form 11-O-formyl-2′-O-acetyl-4″-O-formyl-8,9-anhydroerythromycin A 6,9-hemiketal as a by-product. This by-product is extremely difficultly separated from an objective product, 2′-O-acetyl-4″-O-formyl-8,9-anhydroerythromycin A 6,9-hemiketal, so that there is a problem that the objective product can be difficultly obtained with a high purity. Thus, this method is not suitable for an industrial preparation method of a synthetic raw material of medical product required for purity specification or excellent quality.
To solve the above-mentioned problem, the present inventors have studied extensively to obtain a method of recovering 2′-O-acetyl-4″-O-formyl-8,9-anhydroerythromycin A 6,9-hemiketal which is a final objective product in a state of high purity with substantially not containing a by-product (11-O-formyl-2′-O-acetyl-4″-O-formyl-8,9-anhydroerythromycin A 6,9-hemiketal). As a result, they have found that after hemiketalation by reacting an acid which is after formylation in which a formylating agent is reacted with 2′-O-acetylerythromycin A compound, crystals are precipitated by adding an aqueous basic solution in an aqueous solution, then, crystals containing 2′-O-acetyl-4″-O-formyl-8,9-anhydroerythromycin A 6,9-hemiketal compound with high purity and substantially not containing a by-product can be obtained, whereby they have accomplished the present invention.